Gan semiconductor laser device, and optical disk information system using the laser device

ABSTRACT

Two or more of striped structures  16  and  16 ′ are formed in one chip, and the relaxation oscillation frequency fr 1  of the first striped structure is greater than the relaxation oscillation frequency fr 2  of the second striped structure. The RIN value at low output is improved by the first striped structure having higher relaxation oscillation frequency, and a single transverse mode and reliability at high output are obtained by the second striped structure having lower relaxation oscillation frequency.

FIELD OF THE INVENTION

[0001] The present invention relates to a GaN-based semiconductor laserdevice using a gallium nitride-based compound semiconductor(In_(x)Al_(y)Ga_(1-x-y)N, 0≦x, 0≦y, x+y≦1), and specifically to aGaN-based laser device having multipoint luminescence type comprisingmultiple striped structures in one chip.

DESCRIPTION OF THE BACKGROUND ART

[0002] Recently, with practical application of a GaN type laser diode,the development of optical disc information system which perform highdensity recording to an optical disc by using a light having short wavelength in the range from 380 nm to 430 nm is active. In these opticaldisc systems, reading from an optical disc is carried out at a low powerand writing to an optical disc is carried out at a high output. AGaN-based laser diode having a single striped structure has been used asa light source, and both of reading and writing are carried out bychanging the output of laser diode.

[0003] However, in the case where reading and writing are performed witha typical GaN-based laser diode having a single striped structure, therehas been the following problems when data are transferred at high speedwith a high reading/writing ratio (hereinafter referred to as RW ratio)of laser output. In the optical information disc system, it is necessaryfor reading to keep the value of relative intensity noise (hereinafterreferred to as RIN value) low, and for writing to generate a high outputwith a single transverse mode for a longer operating life. However, itis difficult to keep the RIN value at the level necessary for reading bysimply decreasing output at reading in order to increase the RW ratio,because the RIN value generally decreases as the output decreases. Onthe other hand, it is also difficult to keep the single transverse modeand a long operating life of the laser element by simply enhancing theoutput at writing in order to increase the RW ratio, because the singletransverse mode and the operating life decreases as the output rises.

[0004] Also, in the design of a laser having a striped structure, a highmaximum output can not be obtained when it is designed to decrease thelower RIN value at a low output. On the contrary, in the design toobtain a high maximum output, the RIN value at a low power outputincreases, so that it is difficult to satisfy both of these propertieswith a single laser striped structure.

[0005] For this reason, it is difficult to transfer data at high speedwith a high RW ratio at high speed while keeping a necessary RIN valueand a longer operating life, when a GaN-based laser diode with a typicalsingle striped structure is used to perform both reading and writing.

[0006] It is therefore an object of the present invention is to providea GaN-based semiconductor laser device which can transfer data with highRW ratio at high speed while maintaining necessary RIN value for theoptical disc information system and the longer operating life.

DISCLOSURE OF THE INVENTION

[0007] In order to achieve the above objects, the GaN-basedsemiconductor laser device of the present invention includes a firststriped structure having a first relaxation oscillation frequency fr₁and a second striped structure having a second relaxation oscillationfrequency fr₂ in one chip, and the first relaxation oscillationfrequency fr₁ is greater than the second relaxation oscillationfrequency fr₂.

[0008] Thus, the first feature of the present invention is that two ormore striped structures are formed in one chip, and a striped structureis used for a low power and other striped structure is used for a highpower. By this means an excellent performance at both low and high powercan be obtained. Here, the striped structure may be a striped ridgestructure as shown in the figure, or a striped opening structure such asa current constriction structure. This structure can be applied to anedge emitting laser element having a striped shape waveguide and aplurality of luminous points corresponding to each waveguide. Also, itcan apply to a surface-emitting laser element which has a plurality ofwaveguides and a plurality of luminous points. Moreover, the presentinvention can apply to both free-running oscillation laser element andnon-free-running oscillation laser element having the relaxationoscillation frequency described above.

[0009] The second feature of the present invention is that a stripedstructure having a larger relaxation oscillation frequency fr and astriped structure having a smaller relaxation oscillation frequency frare formed, for the reason that the value of a relaxation oscillationfrequency fr of a laser diode have a large effect on the RIN-Powerproperty. The RIN value at a low output is improved by means of thestriped structure having a higher relaxation oscillation frequency, andthe single transverse mode at a high power and reliability can besecured by means of the striped structure having a smaller relaxationoscillation frequency.

[0010] Therefore, in the case where the GaN-based laser device of thepresent invention is used for an optical disc information system, thefirst striped structure having a higher relaxation oscillation frequencyfr1 is used as a light source for reading, and the second stripedstructure having a smaller relaxation oscillation frequency fr2 is usedas a light source for writing. Besides, in the case where the GaN-basedlaser device of the present invention is used for both of reading andwriting, it is preferable that the first striped structure and thesecond striped structure can be driven independently. It is alsopreferable that the difference of the emission wavelengths between thefirst striped structure and the second striped structure is within 5 nm.

[0011] The relaxation oscillation frequency fr1 of the first stripedstructure and fr2 of the second striped structure can be determined bythe optical wave pattern of relaxation oscillation when a pulsed currentis applied at the same temperature, the same condition, and the sameoutput. FIG. 3 is the graph showing one example of optical wave patternof relaxation oscillation measured under 25° C. with the pulse width of10 ns, the pulse cycle of 20 ns, and the zero bias of 5 mW. When thepeak interval of an optical wave pattern of relaxation oscillation isdetermined as d (s), the relaxation oscillation frequency fr satisfiesfr=1/d (Hz).

[0012] Optimization of the striped structures for a lower output and ahigher output by means of the relaxation oscillation frequency isdescribed in the following. FIG. 4 is the graph showing the relationshipbetween a RIN value and an output P of the GaN-based laser diode at ahigh frequency superimposed drive. As can be seen in FIG. 4, the RINdecreases linearly as the output P of a laser diode gradually increases.However, When the output reaches a certain level, the RIN decreases tothe minimum value, then increases to the maximum value and thendecreases again. Typically, a laser diode is designed for the outputaround or slightly above of the minimum value. However, when theRIN-Power property is optimized according to the high output P2 forwriting (shown as curve 22 in the figure), the RIN value becomes verylarge at the low output P1 for reading. On the other hand, the RIN-Powercurve can be shifted to the lower output side by increasing therelaxation oscillation frequency fr (shown as curve 20 in figure).Therefore, the RIN value for reading can be decreased to a necessarylevel by forming another striped structure having a higher relaxationoscillation frequency fr according to the low output P₁ for reading.

[0013] The RW ratio needed for a general optical disc information systemis about 15 for a normal writing speed. The RW ratio need to beincreased n^(1/2) times when the writing speed is increased n times.Therefore, it is preferable that the difference between the firstrelaxation oscillation frequency fr₁ and the second relaxationoscillation frequency fr2 is greater at higher writing speed. That is, xwhich is defined in x=(fr₁−fr₂)/fr₂ is preferably equal to or greaterthan 0.1, more preferably equal to or greater than 0.3 and furtherpreferably equal to or greater than 0.6.

[0014] The relaxation oscillation frequency can be expressed inEquation 1. In the equation, a is the integral gain, ξ is the opticalconfinement rate, η_(sep) is the front face slope efficiency, P_(out) isthe front face output power, e is the elementary charge, V is the volumeof active region (equal to the product of the cross-sectional area ofthe light emitting region and the length of the resonator), and η₁ isthe internal differential quantum efficiency. $\begin{matrix}{{fr} = \left\lbrack {{\left( \frac{a\quad {\xi\eta}_{sep}p_{out}}{e\quad V\quad \eta_{1}} \right)/2}\pi} \right\rbrack^{\frac{1}{2}}} & \left( {{Equation}\quad 1} \right)\end{matrix}$

[0015] Therefore, the relaxation oscillation frequency fr₁ of the firststriped structure and fr₂ of the second striped structure can becontrolled by adjusting the optical confinement rate of the first andthe second striped structure or the structural parameter of volume V.

[0016] For example, to make the optical confinement rate of the firststriped structure lower than that of the second striped structure(fr₁>fr₂), the average of front face reflectance and back facereflectance of the first striped structure is made larger than that ofthe second striped structure. Besides, the ratio of the equivalentreflective indexes of the wave guide portion to the peripheral portionof the first striped structure may be made greater than that of thesecond structure. For example, in the case where the first and thesecond striped structures have a ridge shape, the ridge depth of thefirst striped structure is made greater than that of the second stripedstructure.

[0017] To reduce the volume V of the active region in the firststructure to less than that of the second striped structure and obtainfr₁>fr₂, the width of the first striped structure may be made narrowerthan that of the second structure, or the resonator of the first stripedstructure may be made shorter than that of the second striped structure.

[0018] It is preferable that the active layers of the first stripedstructure and the second striped structure have the same composition andthe same height so as to simplify the process by forming the activelayers at the same time and control the fluctuation in the emissionwavelength. It is also preferable to form the cladding layers of thefirst striped structure and the second striped structure with the samecomposition and the same height.

[0019] The GaN-based semiconductor laser device may have two or morestriped structures in one chip. However, it is preferable that thestriped structures are formed close to each other to the extent that thestriped structures can be formed and the distance between each center ofthe uttermost striped structures is less than or equal to 150 μm. Thisis because a disturbance occurs in the interaction of the signals to andfrom optical disc information system if the distance between eachstriped structures is greater than 150 μm.

[0020] The optical disc information system according to presentinvention employs a light between 380 and 430 nm for reading from andwriting to an optical disc, and includes the GaN-based semiconductorlaser device whose first striped structure is used as a light source forreading and the second striped structure is used as a light source forwriting. The optical disc information system of the present inventioncan perform data transfer at a high speed with a high RW ratio, whilemaintaining a necessary noise level and life.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a sectional view illustrating a GaN-based semiconductorlaser device according to a first embodiment of the present invention.

[0022]FIG. 2 is a sectional view illustrating a GaN-based semiconductorlaser device according to a second embodiment of the present invention.

[0023]FIG. 3 is a graph showing an example of relaxation oscillationoptical wave pattern.

[0024]FIG. 4 is a graph showing an example of the RIN-Power feature.

[0025]FIG. 5 is a schematic view showing a shape of trench formed in aridged portion.

[0026]FIGS. 6A and 6B are schematic views showing a structure of ridgedportion.

[0027]FIG. 7 is a schematic view showing a structure having resonatorsof different lengths for generating different relaxation oscillationfrequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The preferred embodiments of the present invention will now bedescribed with reference to the accompanying drawings.

[0029] Embodiment 1

[0030]FIG. 1 is a schematic cross-sectional view showing thesemiconductor laser device according to the preferred embodiment 1 ofthe present invention. The semiconductor laser device, shown in FIG. 1is a ridge waveguide GaN-based semiconductor laser device having adouble hetero structure comprising a quantum well active layer 6including In_(x)Ga_(1-x)N(0<x<1) which is formed between an n-typecladding layer 4 including Al_(y)Ga_(1-y)N(0<y<1) and a p-type claddinglayer 8 including Al_(z)Ga_(1-z)N(0<z<1) and also having a stripedstructure formed by making a part of p-type cladding layer a ridgedshape. Two ridges are adjacently formed parallel to each other andconstruct the two striped structures 16 and 16′.

[0031] In the semiconductor laser device shown in FIG. 1, an average ofreflectances of the front end face and back end face in the stripedstructure 16 is about 65 to 85% and an average of reflectance of thefront end face and back end face in the striped structure 16′ is about50 to 60%. For example, a mirror having a reflectance of about 40% isformed only on the part of the front face of the resonator end facecorresponding to the striped structure 16, and no mirror is formed onthe part of the front face of the resonator end face corresponding tothe striped structure 16′. A mirror having a reflectance of about 90% isformed on the back end faces of the striped structure 16, and 16′.

[0032] The Relaxation oscillation frequency f₁ of the striped structure16 which is formed as described above is about 30 to 50% greater thanthe relaxation oscillation frequency f₂ of the striped structure 16′. Asmentioned above, it is advantageous for raising the RW ratio when thedifference in the relaxation oscillation frequencies between the twostriped structures is great. For example, obtaining appropriate RINvalue and life, x, which is described as: x=(fr₁−fr₂)/fr₂, is greaterthan or equal to 0.1, more preferably greater than or equal to 0.3,further preferably greater than or equal to 0.6.

[0033] The RIN-Power characteristic of the striped structure 16 has aminimum value of 1.5 to 2.0 mW as shown in the curve 20 in FIG. 4, andthe RIN-Power characteristic of the striped structure 16′ has a minimumvalue of 4.0 to 5.0 mW as shown in the curve 22 in FIG. 4. Thus, The RINvalue of 125 dB/Hz or less at a low output of about 1.5 to 3.0 mW can beobtained by using the striped structure 16 of semiconductor laser deviceshown in FIG. 1 as a reading light source for an optical discinformation device. Also, a sufficient lifetime at high output of about40 to 70 mW can be obtained by using the striped structure 16′ as areading light source for an optical disc information device.

[0034] The structure of the GaN-based laser device shown in FIG. 1 ishereafter described in detail. GaN is preferably used as the substrate1. However, a foreign substrate different from the nitride semiconductorcan also be used. The foreign substrate may be of a material differentfrom the nitride semiconductor which has been known to be able to growthe nitride semiconductor thereon, such as sapphire having C plane, Rplane or A plane as the principle plane, an insulating substrate such asspinel (MgAl₂O₄), SiC (including 6H, 4H and 3C), ZnS, ZnO, GaAs, Si, anoxide substrate that lattice-matches with the nitride semiconductors, orthe like. Sapphire and spinel are preferable foreign substrates. After abase layer of the nitride semiconductor is formed on the foreignsubstrate prior to forming the device structure, the device structuremay be formed on the single substrate which is made by removing theforeign substrate by a method such as polishing. Or the foreignsubstrate may be removed after the device structure is formed.

[0035] The n-type nitride semiconductor layer of the n-type contactlayer 2 and the n-type cladding layer 4 are formed on the substrate 1via a buffer layer (not shown). A n-type optical guide layer can beformed between the n-type cladding layer 4 and the active layer 6. Atleast the part of the n-type semiconductor layer which is made to be incontact with the active layer 6 is needed to have a wider band gap thanthat of the active layer 6, so that the composition including Al ispreferable. Moreover, each layer can be grown to n-type with doping ann-type impurity, or without doping impurities.

[0036] The active layer 6 is formed on the n-type semiconductor layers 2to 4. The active layer 6 has a MGW structure where a well layer of InGaN(0<x₁<1) and a barrier layer of InGaN (0≦x₂<1, x₁>x₂) are repeatedlyformed to an appropriate number of times. Both ends of the active layerare barrier layers. The well layers are formed without doping and allbarrier layers are doped with an n-type impurity such as Si and Sn witha preferable concentrations of 1×10¹⁷ to 1×10 ¹⁹ cm⁻³.

[0037] The p-type cladding layer 8 is formed as the p-type nitridesemiconductor layer on the final barrier layer. The p-type electronconfinement layer or the p-type optical guide layer can be formedbetween the p-type cladding layer 8 and the active layer 6. At least thepart of the p-type cladding layer 8 which is made contact with activelayer is needed to have wider band gap than that of the active layer 6,so that the composition including Al is preferable.

[0038] Two ridge stripes 8 a and 8 a′ are formed halfway in the p-typecladding layer 8 in the case where the p-type optical guide layer isformed to the halfway thereof, and two striped structures 16 and 16′ areconstructed. The device layers such as the n-type cladding layer 4, theactive layer 6, and the p-type cladding layer 8 are common in thestriped structures 16 and 16′. Two ridge stripes 8 a and 8 a′ are formedin the same shape and parallel to each other. The distance d between thecenters of the ridge stripes is preferably 150 μm or less, and morepreferably 70 μm or less. When the distance d between the centers isshorter, the shifts of the luminous points between reading and writingcan be made smaller. The p-type electrodes 12 and 12′ and the p-type padelectrodes 14 and 14′ are formed on each of two ridge stripes 8 a and 8a′. The p-type electrodes 12 and 12′ and the p-type pad electrodes 14and 14′ are formed separately on each striped structure so as to drivethe striped structures 16 and 16′ independently.

[0039] Furthermore, the n-type electrode 10 is formed on the surface ofthe n-type contact layer exposed by removing the upper layers. Then-type electrode 10 is common to the striped structures 16 and 16′. Asmentioned above, in the case where the p-type electrode and the n-typeelectrode are formed on the same face of the substrate and the n-typeelectrode is common to two striped structures, the physicalrelationships of the n-type electrode 10, the striped structure 16 forreading, and the striped structure 16′ for writing are not specificallylimited, however, it is preferable that the common n-type electrode 10is formed adjacent to the striped structure for writing where arelatively large current is applied. With this structure, the luminousefficiency can be improved by reducing the current path to the stripedstructure 16′ for writing. Meanwhile, the structure of the n-typeelectrode is not specifically limited and the n-type electrode can beformed on each striped structure independently or on the rear faceinstead of the same face side of the substrate.

[0040] A mirror having a reflectance of about 90% is formed on thealmost entire area of the back end face of the resonator (monitoringface) of the obtained striped structures 16 and 16′. A mirror having areflectance of about 40% is formed only on the front end face of theresonator (light emitting face) on the side of the striped structure 16.The end face formed by cleaving or etching can be used for the part onthe front face of the resonator corresponding to the striped structure16′. The materials used for the mirror are not specifically limited andSiO₂, Al₂O₃, TiO₂, ZrO₂ or the like can be used.

[0041] In this embodiment, the optical confinement rate between thestriped structures 16 and 16′ is adjusted by the presence of the mirroron the front end face of the resonator, however the mirror constructionis not specifically limited if the average of the reflectances of thefront and back end faces can be adjusted to the required ratio betweenthe striped structures. For example, the mirrors may be formed on theboth front and back faces of the both striped structures with only themirror reflectances different from one another.

[0042] Embodiment 2

[0043]FIG. 2 is a schematic sectional view of the GaN-basedsemiconductor laser device according to the embodiment 2. In thisembodiment, the n-electrodes 10 and 10′ are formed independently on eachstriped structure and the trench 11 are formed between the stripedstructures through all the element layers from the n-type contact layer2 to the p-type cladding layer 8. The other processes are performed inthe same manner as in Example 1.

[0044] According to the current embodiment, the overlapping of thecurrent path in the striped structures for reading 16 and the stripedstructure for writing 16′ can be completely eliminated, so that thenoise which appears in the laser beam with simultaneous drive can begreatly reduced. Also, as shown in FIG. 5, the width d′ of the trench 11can be widen to d″ in the vicinity of the luminous points 7, in order tocontrol the noise more effectively.

[0045] Embodiment 3

[0046] In the embodiments 1 and 2, the relaxation oscillation frequencyis adjusted by changing the construction of the resonator mirror. Inthis embodiment, the relaxation oscillation frequency is adjusted bychanging the width and the depth of the ridge parts in stripedstructures. The mirror construction is common to the striped structuresand the other processes are performed in the same manner as embodiment 1and 2, except that the shapes of the ridge parts 8 a are different fromeach other.

[0047]FIG. 6A is a schematic view showing the shape of the ridge part ofthe p-type cladding layer 8. The widths W and the depths D of the ridgeparts are made to differ in the striped structures 16 and 16′. That is,the shapes of the ridge part 8 a in the stripe structure 16 for readingis adjusted so that the width W is narrower and the depth D is deeperthan in the striped structure 16′. The volume V of the active region canbe reduced by narrowing the width W and the difference in the refractiveindexes Δn in the lateral direction in the ridge parts become grater bydeepening the depth D, so that the optical confinement rate ξ can beraised. Thus, as shown in an above-mentioned equation 1, the relaxationoscillation frequency fr₁ of the striped structure 16 can be madegreater than the relaxation oscillation frequency fr₂ of the stripedstructure 16′.

[0048] Furthermore, as shown in FIG. 6B, in order to increase thedifference in the optical confinement rate ξ between the stripedstructures, the ridge portions 8 a of the striped structures can beembedded with a different material 9, and the presence and the materialof the embedding material may be changed between the striped structures.

[0049] Embodiment 3

[0050] In the embodiments 1 and 2, the relaxation oscillation frequencyis adjusted by changing the mirror structures of the resonator. In thisembodiment, the relaxation oscillation frequency is adjusted by changingthe resonator lengths between striped structures. The mirror structureis common to the striped structures and the other processes areperformed in the same manner as embodiment 1 and 2 except that theresonator lengths 8 a differ between the striped structures.

[0051]FIG. 7 is schematic view showing the striped structures formednext to each other. The resonator lengths are made to differ between thestriped structures 16 and 16′. That is, the resonator length L₁ ofstriped structure for reading 16 is made shorter than the resonatorlength L₂ of the striped structure 16′. The volume V of the activeregion in the striped structure 16 can be made smaller than that of thestriped structure 16′ by making the resonator length L₁ shorter. Thus,as shown in the above-mentioned equation 1, the relaxation oscillationfrequency fr₁ of the striped structure 16 can be made greater than therelaxation oscillation frequency fr₂ of the striped structure 16′.

[0052] In the embodiments 1 to 4, the processes for varying therelaxation oscillation frequencies between the striped structures bychanging the mirror structures, the ridge shapes, and the resonatorlength are explained. These processes may not only be employedindependently but also are properly combined. All other possibilitiesare accepted to use if they are the means to vary the parameters showedin equation 1, that is, the integral gain a, the optical confinementrate ξ, the front face slope efficiency η_(sep), the front face outputpower P_(out), the volume of the active region V and the internaldifferential quantum efficiency η₁ between the striped structures.

[0053] In the embodiments 1 to 4, only the constructions with twostriped structures are described, however, three or more stripedstructures may be formed with different relaxation oscillationfrequencies according to the properties needed respectively.

[0054] The example related to present invention will be described below.

EXAMPLE 1

[0055] A GaN-based semiconductor device having the structure, shown inFIG. 2, is formed as described below.

[0056] (Substrate)

[0057] A foreign substrate which is made of sapphire having c-plane asthe principal plane and the diameter of 2 inches is set in a MOVPEreaction vessel. The temperature is adjusted to 500° C. and a bufferlayer made of GaN is formed to the thickness of 200 Å, by usingtrimethyl gallium (TMG) and ammonia (NH₃). Then, the temperature israised and a foundation layer made of undoped GaN is grown to thethickness of 1.5 μm. A plurality of striped masks is formed on thesurface of the foundation layer, and a nitride semiconductor, GaN inthis example, is selectively grown from the opening of the masks (windowportion). Thus, the nitride semiconductor substrate having a nitridesemiconductor layer which is formed with the lateral growth (ELOG) isobtained. In this instance, the mask used in the selective growth iscomprised of SiO₂ with the mask width of 15 μm and the opening width(window portion) of 5 μm.

[0058] (Buffer Layer)

[0059] After forming the buffer layer on the nitride substrate, thetemperature is adjusted to 1050° C. and the buffer layer comprisingAl_(0.05)Ga_(0.95)N is formed to the thickness of 4 μm by using TMG(trimethyl gallium), TMA (trimethyl aluminum), and ammonia. This layerfunctions as the buffer layer between the n-type contact layercomprising AlGaN and the nitride semiconductor substrate comprising GaN.Next, the each layer that becomes a device structure is laminated on theunderlayer that comprising the nitride semiconductor.

[0060] (N-Type Contact Layer 2)

[0061] Then, an n type contact layer 2 comprising Al_(0.05)Ga_(0.95)Ndoped with Si is formed to the thickness of 4 μm at 1050° C. on theobtained buffer layer, by using TMG, TMA, ammonia, and silane gas as theimpurity gas.

[0062] (Crack Prevention Layer)

[0063] Then, a crack prevention layer comprising In_(0.06)Ga_(0.94)N isformed to the thickness of 0.15 μm at 800° C. by using TMG, TMI(trimethl indium), and ammonia.

[0064] (N-Type Cladding Layer 4)

[0065] Next, the temperature is adjusted to 1050° C., and the A layercomprised of undoped Al_(0.05)Ga_(0.95)N is grown to the thickness of 25Å by using TMA, TMG and ammonia as the source gas. The supply of TMA isthen stopped and the B layer comprising GaN doped with Si inconcentration of 5×10¹⁸/cm³ is grown to the thickness of 25 Å by usingsilane gas as the impurity gas. Then, n-type cladding layer consists ofa multiple layer (super lattice structure) with the total thickness of 1μm is formed by repeating each of the above operations 200 times tolaminate the A layer and the B layer.

[0066] (N-Type Optical Guide Layer)

[0067] Then, at the same temperature, an n-type optical guide layercomprising undoped GaN is grown to the thickness of 0.15 μm by using TMGand ammonia as the source gas. Besides, an n-type impurity can be doped.

[0068] (Active Layer)

[0069] Then, the temperature is adjusted to 800° C. The barrier layer(B) comprising In_(0.05)Ga_(0.95)N doped with Si in a concentration of5×10¹⁸/cm³ is grown to the thickness of 140 Å by using TMI, TMG, andammonia as the source gas, and silane gas as the impurity gas.

[0070] The supply of silane gas is then stopped, and the well layer (w)comprising undoped In_(0.1)Ga_(0.9)N is grown to the thickness of 55 Å.This barrier layer (B) and the well layer (w) are laminated in sequenceof (B)/(W)/(B)/(W). Finally, undoped In_(0.05)Ga_(0.95)N is grown as abarrier layer by using TMI (trimethl indium), TMG and ammonia as thesource gas. The active layer becomes the multiquantum well structure(MQW) of total thickness of about 500 Å.

[0071] (P-Type Electron Confinement Layer)

[0072] Next, the p-type electron confinement layer comprisingAl_(0.3)Ga_(0.7)N doped with Mg in a concentration of 1×10¹⁹/cm³ isgrown to The thickness of 100 Å by using TMA, TMG and ammonia as thesource gas and Cp₂Mg (cyclopentadienyl magnesium) as the impurity gas.

[0073] (P-Type Optical Guide Layer)

[0074] Then, the temperature is adjusted to 1050° C. and the p-typeoptical guide layer comprising undoped GaN is grown to the thickness of0.15 μm by using TMG and ammonia as the source gas. This p-type opticallayer is grown as an undoped layer, however, the diffusion of Mg fromthe adjacent layers such as the p-type optical guide layer 108 and thep-type cladding layer 109 increases the Mg concentration to 5×10¹⁶/cm³and turns to be of the p-type.

[0075] (P-Type Cladding Layer 8)

[0076] Subsequently, a layer comprising undoped Al_(0.05)Ga_(0.9) 5N isformed to the thickness of 25 Å. The supply of TMA is then stopped, anda layer comprising Mg doped GaN is grown to the thickness of 25 Å usingCp₂Mg. The p-type cladding layer 8 made of the super lattice structurewith the total thickness of 0.45 μm is grown by repeating thisoperations 90 times.

[0077] (P-Type Contact Layer)

[0078] Finally, the p-type contact layer made of the p-type GaN dopedwith Mg in the concentration of 1×10²⁰/cm³ is grown to the thickness of105 Å on the p-type cladding layer 8 at 1050° C. After the reactionends, annealing is carried out to the wafer in the reaction vessel inthe nitrogen atmosphere at 700° C., so as to further lower theresistance of the p-type layer.

[0079] After growing the nitride semiconductor and laminating each layeras described above, etching is carried out by way of RIE (reactive ionetching) with Cl₂ gas, so as to expose the surface of the n-type contactlayer 2 whereon the n-electrode to be formed, as shown in FIG. 2. At thesame time, the front end face and the back end face of the etching endfaces are formed to become the resonator face. In the vicinity of thecenter of the element, the trench 11 which reaches substrate 1 is formedwith the width of 20 μm. The trench 11 is further expanded to the widthof 40 μm to reach buffer layer, as described in FIG. 5. As justdescribed, SiO₂ is the best as a protective film for deep etching of anitride semiconductor.

[0080] Next, the striped ridges 8 a and 8 a′ are formed on the oppositeside of the trench 11 as the striped waveguide region. An oxide of Si(mainly SiO₂) is used for the protective film for forming the stripedridge. The width W of the top surface of the ridge stripe is 1.8 μm, thedepth D of the ridge is 0.5 μm, and the distance between the centers ofthe two ridge stripes is 50 μm. Here, the depth of the striped ridge isthe depth to the remaining portion of the p-type optical guide layer, towhere etching is carried out through the p-type contact layer and thep-type cladding layer.

[0081] After the striped ridges are formed, the protective film 15 madeof an Zirconium oxide (mainly ZrO₂) is disposed on the sides of thestriped ridges and the continuous plane thereof except on the trench 11and the striped ridges 8 a and 8 a′.

[0082] Then, the p-electrodes 12 and 12′ comprising Ni/Au are formed onthe top surface of the p-type contact layer in the exposed portion ofthe striped ridge, On the already exposed top surface of the n-typecontact layer 2, the striped n-type electrodes 10 and 10′ are formedalong the striped ridge.

[0083] Next, a pad electrode comprising Ni—Ti—Au (1000 Å-1000 Å-8000 Å)is formed on the p electrode, and the same pad electrodes are alsoformed on the n-electrode.

[0084] On the etching end faces which are formed by exposing the n-typecontact layer, a mirror with the reflectance of 40% on the front endface of the stripe 16, and mirrors with the reflectance of 90% on theback end faces of the stripes 16 and 16′, are respectively formed by wayof photolithography.

[0085] After the n-electrode and the p-electrode are formed as describedabove, the wafer is divided into bars outside of the end face on theM-face (the M-face of GaN, such as (11-00)) in the directionperpendicular to the striped electrodes, so as to obtain the laserdevice whose resonator length is 650 μm.

[0086] In the stripes, where the mirror was formed on the front end faceof the resonator was assumed to be the stripe for reading 16, and wherethe mirror was not formed on the front end face of the resonator wasassumed to be the stripe for writing 16′, and the characteristics wereevaluated. When the relaxation oscillation frequency are measured underthe condition of 25° C., the pulse width of 10 ns, the pulse cycle of 20ns, and the zero bias of 5 mW, the relaxation oscillation frequency fr₁of the stripe 16 for reading was 1.9 GHz and the relaxation oscillationfrequency fr₂ of the stripe 16′ for writing was 1.4 GHz.

[0087] In the stripe for reading 16, the RIN value was −128 dB/Hz andthe slope efficiency was 0.6 W/A at the output of 2.0 mW, and the outputwhere the RIN value was −118 dB/Hz was 1.5 mW. On the other hand, in thestripe for writing 16′, the RIN value was −118 dB/Hz and the slopeefficiency was 1.2 W/A at the output of 2.0 mW, and the output where theRIN value was −125 dB/Hz was 4.0 mW.

[0088] When the obtained GaN-based semiconductor laser device was usedin an optical disc information device and the output of the stripe forreading 16 was adjusted to 2.0 mW and the output of the stripe forwriting was adjusted to 30 mW, the operation life was 5000 hours andcontrolled by the stripe for writing.

EXAMPLE 2

[0089] The GaN-based semiconductor laser device obtained in example 1was used at the double speed rate for writing in an optical discinformation system. When the output of the stripe 16 for writing was 2.0mW and the output of the stripe 16′ for reading was 45 mW, the operatinglife was 2500 h because the rate was controlled by the stripe forwriting.

COMPARATIVE EXAMPLE 1

[0090] Only the stripe for reading 16 in the example 1 was used for anoptical disc information system and both of reading and writing wereoperated using one stripe. To adjust the RIN value at the time ofreading to −125 dB/Hz, the output for reading was adjusted to 4.0 mW andthe output for writing was adjusted to 60 mW. The life was 1500 hour inthis condition, which was less than one third compared with example 1.When the output for writing was adjusted to 84 mW in order to use at thedouble speed rate in writing, the life was 1000 h, which was less thanhalf compared with example 2.

COMPARATIVE EXAMPLE 2

[0091] Only the stripe for writing 16 in example 1 was used for theoptical disc information system and both of reading and writing wereoperated using one stripe.

[0092] When the output for reading was adjusted to 2.0 mW and the outputfor writing was adjusted to 30 mW, the life was 1000 h, which was aboutone fifth compared with example 1.

[0093] The GaN-based semiconductor laser device according to the presentinvention includes two or more striped structures formed in one chip,wherein one stripe is used for lower output and another stripe is usedfor higher output. Therefore, an excellent performance can be obtainedat both lower output and higher output. Furthermore, a striped structurehaving a higher relaxation oscillation frequency fr and a stripedstructure having a lower relaxation oscillation frequency fr are formed.Therefore, the RIN value at lower output can be improved by the stripedstructure having higher relaxation oscillation frequency, and the singletransverse mode and reliability at higher output can be obtained by thestriped structure having lower relaxation oscillation frequency. Thus,by using the GaN-based semiconductor laser device related to the presentinvention in an optical information system, data can be transferred athigh speed with high RW ratio, while maintaining the required level ofRIN value and life.

[0094] It is to be understood that although the present invention hasbeen described with regard to preferred embodiments thereof, variousother embodiments and varients may occur to those skilled in the art,which are within the scope and spirit of the invention, and such otherembodiments and varients are intended to be covered by the followingclaims.

1-14. (Canceled)
 15. A GaN-based semiconductor laser device comprising,in one laser chip, a first striped structure having a first relaxationoscillation frequency fr₁ and a second striped structure having a secondrelaxation oscillation frequency fr₂, wherein the length of said firststriped structure is shorter than that of said second striped structureand an average of the front-face reflectance and the back-facereflectance of said first striped structure is greater than that of saidsecond striped structure so that the first relaxation oscillationfrequency fr₁ is made greater than the second relaxation oscillationfrequency fr₂.
 16. The GaN based semiconductor laser device according toclaim 15, wherein p-electrodes are formed on said first stripedstructure and said second striped structures, respectively; ann-electrode, which is on the same side as said p-electrodes and iscommon to said first and second striped structures, is formed in one oftwo regions that sandwich said first and second striped structures; andsaid n-electrode is adjacent to said second striped structure.
 17. TheGaN based semiconductor laser device according to claim 15, wherein atrench is formed between said first striped structure and said secondstriped structure, and the width of said trench is partly broadened neara luminous region of said laser device.
 18. The GaN-based semiconductorlaser device according to claim 15, wherein the difference in wavelengthbetween light emitted from said first striped structure and that fromsaid second striped structure is within 5 nm.
 19. The GaN-basedsemiconductor laser device according to claim 15, wherein the firstrelaxation oscillation frequency fr1 and the second relaxationoscillation frequency fr₂ satisfy the relational expression of(fr₁−fr₂)/fr₂>0.1.
 20. The GaN-based semiconductor laser deviceaccording to claim 15, wherein the ratio of equivalent refractiveindexes between a waveguide portion and a peripheral portion of saidfirst striped structure is greater than that ratio of said secondstriped structure.
 21. The GaN-based semiconductor laser deviceaccording to claim 15, wherein at least one of end faces of said firstand second striped structures is formed by etching.